1. Field of the Invention
The present invention relates to a forced opening and closing valve operating system in which an engine valve is forcibly operated during valve opening and valve closing, and more particularly to a forced opening and closing valve operating system in which the operating characteristics of an engine valve are changed according to low speed running conditions and high speed running conditions of the engine.
2. Description of Related Art
A forced opening and closing valve operating system in which the operating characteristics of an engine valve are changed according to low speed running conditions and high speed running conditions of the engine is already known from, for example, Japanese Patent Application Laid-open No. 5-321617.
In this conventional forced opening and closing valve operating system, a rocker shaft having two different eccentric portions is supported in a cylinder head so that the rocker shaft can undergo angular displacement around its axis, and a low speed opening rocker arm is connected to an engine valve which is either an intake valve or an exhaust valve, so as to exert on the engine valve a driving force in a valve-opening direction, the low speed opening rocker arm being supported on a non-eccentric portion of the rocker shaft so as to swing by following a low speed valve-opening cam.
Further, a low speed closing rocker arm is connected to the engine valve so as to exert on the engine valve a driving force in a valve-closing direction, the low speed closing rocker arm being supported on one of the eccentric portions of the rocker shaft so as to swing by following a low speed valve-closing cam. Both a high speed opening rocker arm which can swing by following a high speed valve-opening cam and can exert a force in a valve-opening direction on the low speed opening rocker arm while swinging, and a high speed closing rocker arm which can swing by following a high speed valve-closing cam and can exert a force in a valve-closing direction on the low speed closing rocker arm while swinging, are supported on the other one of the eccentric-portions of the rocker shaft. The rocker shaft undergoes angular displacement so that the rocker arms are made to come into sliding contact with the corresponding cams or separate therefrom, thus forcibly opening or closing the engine valve while changing the valve operating characteristics according to the running conditions of the engine.
However, in such a conventional forced opening and closing valve operating system, it is necessary to secure a space in the cylinder head for the rocker arms to come into sliding contact with the corresponding cams or separate therefrom for the angular displacement of the rocker shaft, and the dimensions of the cylinder head increase accordingly. Furthermore, in order to make the operating characteristics of the engine valve change according to low speed running conditions and high speed running conditions of the engine, at least four rocker arms are required, that is, the number of components cannot be said to be small, and it is necessary to secure a space for these rocker arms to be arranged along the axial direction of the rocker shaft, which also increases the dimensions of the cylinder head. Moreover, noise, such as knocking sound, is caused when the rocker arms come into sliding contact with and separate from the corresponding cams.